High Alloy Iron, Use of the Material for Structural Components that are Subject to High Thermal Stress and Corresponding Structural Component

ABSTRACT

The invention relates to a high alloy iron that has an austenitic structure and a spherical graphite configuration and comprises the following components in percent by weight: carbon&lt;2.0%, silicon 2.0-6.0%, nickel 29-36%, chrome 1.0-2.5%, niobium 0.1-1.0% and molybdenum 0.1-2.5%. The invention also relates to the use of the cast iron material for structural components that are subject to high thermal stress, especially for exhaust manifolds and turbocharger housings of internal combustion engines of the individual or integrated type of construction. The invention finally relates to a structural component that is subject to high thermal stress, especially the exhaust system of an engine consisting of the aforementioned cast iron material.

The present invention refers to a high-alloy cast iron material with anausthenitic structure and a spherical graphite configuration. Thestructure according to the invention is especially suitable for use inthose parts of an engine that are under great thermal stress,particularly the exhaust system of an engine, but is also suitable forother structural components under high thermal stress.

The thermal stresses of exhaust manifolds and turbocharger housings haveincreased a great deal due to the introduction of new burning processes.The exhaust temperatures of modern Otto engines, in particular,currently reach well over 1000° C. Structural components subject to suchthermal stresses must have, apart from high-temperature resistance, highscaling resistance, good temperature fluctuation resistance, and a lowtemperature expansion coefficient. Therefore, it is known from the stateof art that structural components under high thermal stress are made ofcast steel. Although the latter satisfies the requirements listed above,it is very expensive and therefore not very suitable for producing suchstructural components in series.

Furthermore, austhenitic cast iron alloys with spherical graphite areknown from technical advances under the trade name Ni-Resist. Thesealloys, described in EN 13835 as EN-GJSA XNi 35 (or according to ASTM439 as D5), have high thermal resistance, good scaling resistance andvery high temperature fluctuation resistance in addition to their goodmechanical properties. That is why in the state of art they are used forseries production of engine parts under high thermal stress,particularly for exhaust manifolds and/or turbocharger housings ofhighly compressed or supercharged engines.

These alloys are economical and can be easily cast. They already havevery high thermal resistance of up to 1050 degrees centigrade, asindicated in EN 13835. However, in thin-walled structural componentswith wall thicknesses lower than 8 mm, thermal resistance falls to onlyabout 950° C. In order to lower temperatures to this maximum permissiblerate, injection and burning are carefully regulated by expensive enginecontrol, which in turn uses up once again more fuel. Therefore, there isa need to develop materials that will have higher resistance under hightemperature conditions even if their components have thin walls.

The task of the present invention is to suggest an austhenitic cast ironmaterial with very high thermal resistance in spite of havingthin-walled structural components.

The task is solved with a cast iron material, an application thereof,and with structural components in accordance with the features of theindependent claims.

According to the invention, a high-alloy cast iron material having anausthenitic structure and a spherical configuration contains, amongother things, the following elements (in % of weight): carbon<2%,silicon 2.0-6.0%, nickel 29-36%, chrome 1.0-2.5%, niobium 0.1-1.0%, andmolybdenum 0.1-2.5%. Compared to known austhenitic alloys containingspherical graphite based on nickel, molybdenum and niobium, inparticular, have been added to the material in alloyed form in theconcentrations given above. It has been shown that by adding molybdenumtogether with niobium, one can increase the temperature fluctuationresistance of such alloys, without causing the material to becomebrittle by the formation of carbide. In addition, the resistance of thematerial under higher temperatures and the scaling resistance can beimproved with an alloy according to the invention. In this case, thealloy according to the invention has a relatively low thermal expansioncoefficient, thus reducing the cracking risk that can occur undertemperature fluctuation conditions.

The alloy's carbon concentration has been limited to a value of 2% toprevent carbide formation. Here, the carbon concentration is adjusted insuch a way that in spite of this, the molten mass still has good flowingand pouring properties.

It is known that the alloy's silicon concentration has a deoxidizingeffect and improves here the resistance against hot gas corrosion.Nevertheless, the material according to the invention has anadvantageously low thermal expansion coefficient.

Chrome improves oxidation resistance under high temperatures. Here, theproportion of chrome has been limited to 2.5%.

A particularly advantageous configuration of the invention provides thecast iron material to have a manganese concentration of 0.5-1.5%. Themanganese concentration in an alloy influences on the one hand thepouring properties, but on the other hand reduces graphite precipitationas well, so that the manganese concentration in this case has beenpreferably limited to 1.5%.

In accordance with another advantageous configuration of the invention,the material has a phosphorous concentration lower than 0.1%, since ahigher proportion of phosphorous could also lead to the material'sbrittleness. Furthermore, it is advantageous for the material to have acopper concentration of less than 0.5%.

An especially preferred configuration of the invention foresees the castiron material to have a nickel concentration of 34 to 36%. Thisproportion of nickel has the function of creating an austhenitic basicstructure.

Another advantageous configuration of the invention provides thematerial to have preferably a chrome concentration of 1.5-2.5% in orderto improve both high-temperature resistance and oxidation resistance.

It is particularly advantageous for the cast iron (in accordance withthe present invention) to have a higher temperature resistance comparedwith the conventional, austhenitic cast iron alloys that use sphericalgraphite and nickel as main carriers (D5). Therefore, the cast ironaccording to the invention is also highly suitable for use in extremelythin-walled materials having wall thicknesses of 3-6 mm under very hightemperatures.

The material according to the invention is therefore extremely suitablefor use in structural components under high thermal stress. Thethin-walled structural components made from the cast iron materialaccording to the invention are resistant to about 985 degrees centigradeaccording to current knowledge. According to the invention, thishigh-alloyed cast iron material is therefore used for parts that make upthe exhaust system of an engine. The material is especially suitable forexhaust manifolds and turbocharger housings. The material can be usedwith special advance in parts of the exhaust system of highly compressedand supercharged engines, in which exhaust temperatures of up to 1100°C. occur.

The material is particularly suitable for use in integral housings (inother words, housings that integrate exhaust manifolds and turbinehousings of exhaust turbochargers).

According to the invention, structural components under high thermalstress (especially those of an engine's exhaust system) are composed ofa cast iron material having the properties described above. Therefore,the cast iron used in the present invention can replace the materialsthat are conventionally used for exhausts systems. By and large, thecast iron can be manufactured and processed in accordance with the usualmethods. The structural components can be subject to an annealingtreatment for homogenizing the structure, which in turn achieves bettercarbide distribution.

Compared to conventional austhenitic cast iron alloys for use in vehicleexhaust systems it was possible to achieve an increase of thermalresistance by about 35 degrees centigrade. The increase in the exhaustor burning temperature achieved as a result of this also allows one toincrease displacement-related power and reduce contaminant emissions,thus conserving fuel.

In addition, the cast iron material according to the invention had an 8%higher temperature fluctuation resistance in the shear-crack testcompared to conventional alloys belonging to the D5 class (ASTM 439). Inthe autobahn permanence test, improvements of up to 30% could be seenwith the material according to the invention.

The invention is especially suitable for structural components of avehicular exhaust system, but not limited to them. Modifications andcombinations within the scope of the invention also fall under thepatent claims.

1. High-alloy cast iron material having an austhenitic structure andspherical graphite configuration with higher temperature fluctuationresistance and advantageous use for engines characterized in that thematerial expressed in percent of weight contains the followingingredients: Carbon <2.0% Silicon 2.0-6.0% Nickel 29-36% Chrome 1.0-2.5%Niobium 0.1-1.0% Molybdenum 0.1-2.5%

2-13. (canceled)